The present invention relates to a thermocouple linearizer circuit for use in a device for digitally displaying the temperature of a thermocouple.
A thermocouple is a device that uses the voltage developed by the junction of two dissimilar metals to measure temperature differences. Two wires of dissimilar metals welded together at one end make up the basic thermocouple. This junction, called the sensing or measuring junction, is placed at the point where the temperature is to be measured. The other junction, called the reference or cold junction, is maintained at a known reference temperature. The voltage developed between the cold junction ends depends on the difference between the temperatures of the two junctions.
The prior art has generally not been required to be accurate, stable or as repeatable as is now required for digital display applications using thermocouples.
In order to display temperatures, previous methods have included the use of a diode switch inside an amplifier, but the temperature stability of diodes generally limits the accuracy when used with a device for digitally displaying the temperature.
Another method is digital linearization, in which an actual response curve representing the particular thermocouple temperature is digitized and stored in a random access memory. Digital linearization generally provides acceptable accuracy for a digital display but has the disadvantage of requiring sufficient memory capacity to properly represent the temperature over the range of the particular thermocouple, resulting in a corresponding increase in the cost of the device.
Another method is to incorporate multiplicative correction of the slope of the response curve of a thermocouple, which is initiated generally at low levels in a feedback loop of an input amplifier and using diodes as switching or non-linear elements. This method is described in an article entitled "Thermocouple Output Voltage Linearizer" by A. J. Haywood, Electronic Engineering, November, 1974, p. 36. However, a problem with multiplicative techniques is that a correction of the thermocouple by multiplication at a low level can result in multiplication of any random error signal, thereby decreasing the accuracy.
In view of the above background, it is an objective of the present invention to provide an improved thermocouple linearizer circuit which is relatively inexpensive, requires few parts, is quickly adjustable, and is stable over the broad temperature range for a number of thermocouples and provides the necessary accuracy for applications using thermocouples such as for a digital display.